An electric motor can be triggered in almost stepless fashion with the aid of a pulse width modulation signal. The pulse width modulation signal is applied for that purpose to a switching device that preferably encompasses a field-effect power transistor, in order to apply the supply voltage to an electric motor as a function of the pulse width modulation signal.
The pulse width modulation signal has a triggering frequency and a pulse duty factor. The pulse duty factor determines the degree of triggering of the electric motor, and is defined as the ratio between the time during which the electric motor is switched on, and the period of the triggering frequency.
When selecting the triggering frequency, particular care must be taken that the selected frequency is not in the audible region, since troublesome audible oscillations of the electric motor or the triggering circuit may otherwise occur.
The periodic switching of the supply voltage by the electric motor that is performed in accordance with the pulse width modulation signal results in periodic voltage fluctuations on the supply lines, which can have an interfering effect on the supply voltage system. For this purpose, a low-pass filter circuit is usually provided in the supply voltage system in order to filter out to the greatest possible extent, and thus smooth, high-frequency voltage fluctuations, i.e. voltage fluctuations in the region of the triggering frequency and higher. This is not, however, completely achieved by a low-pass filter circuit.
The switching characteristic curve results in abrupt switching, especially because of the steep triggering edges at a gate input of the field-effect power transistor, so that in addition to the frequency component of the triggering frequency, a large component of higher-frequency harmonics also occurs in the supply voltage line. The higher the frequency, the more disruptive these are in the supply voltage system.
It is the object of the present invention to provide a method and a control circuit in order to reduce line-conducted interference in a pulse width modulated triggering system of an electric motor.